Southern Oregon's Mount Fuji soars

Friday

Sep 20, 2013 at 2:00 AM

Mount McLoughlin is our closest High Cascade volcano. As seen from our valley, it's a symmetrical, Mount Fuji-like cone (in winter looking like a bald eagle's head) with two small satellite vents on its sides.

By Jad D'Allura

Mount McLoughlin is our closest High Cascade volcano. As seen from our valley, it's a symmetrical, Mount Fuji-like cone (in winter looking like a bald eagle's head) with two small satellite vents on its sides.

Yet from Four Mile Lake, it looks as if a giant ice cream scooper gouged out its eastern side. What's up with that?

The volcano was active between 1.9 million and 50,000 years ago. It's an unusual volcano in that its core doesn't consist of alternating volcanic debris and harder "corseting" lava flows but a huge cinder cone that rose about 3,000 feet.

That's very unusual for volcanoes made of crumbly cinders. The next phase of volcano-building was dominated by lava flows that issued from the main central vent, satellite vents, and from the volcano's base. The andesite lava comprises blocky flows, some forming the aquifers of Medford's main water supply, Big Butte Springs. Water from snowmelt and rainfall not only runs along the surface but seeps into cracks in the lava to form pure groundwater aquifers. Lava eruptions continued through the Ice Age, flowing over early glacial debris, and possibly continued to about 1,500-2,000 years ago.

The present peak isn't the top of the original volcano. Mount McLoughlin was at the southern terminus of an icecap that almost covered the High Cascade volcanoes. Up to 500 feet of the peak was gnawed away by intense glacial activity. The ice carved a semi-circular basin ("cirque") out of the east side of the volcano. A century ago, a small glacier, now long-vanished, occupied that cirque. The most recent deposits are sweeping debris avalanches that course down the steep east side.

Visit our Mount Fuji during the summer. For those in shape, the six-mile trail to the top of the 9,493-foot summit forms a relatively easy, most-of-the-day hike, but it isn't suggested for the doughnut-impaired. Take plenty of water (there's no Sno-cone concession at the top).

The upper trail is poorly marked, so pick an identifiable feature once leaving tree line and look back often to mark its location. Basaltic andesite boulders show black iron-pyroxene and whitish plagioclase crystals in a fine-grained matrix. Once at the top, look for fulgurite, rock glass produced by lightning strikes. As seen near the remains of an old fire lookout, fulgurite appears as dark green smears that resemble thick, glassy paint.

Look to the south to see the stark volcanic field of Brown Mountain, an andesitic shield volcano with a very small cinder cone at its top, and the double-humped Mount Shasta in the distance.

For maps, photos, stories and details on how to climb Mount McLoughlin, see www.mailtribune.com/McLoughlin

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.